Proceeding Articles

A digital computer was installed on the Centre Technique du Papier’s experimental machine in order to study the problems involved in the automation of a papermachine.

After a short description of the installation, this paper summarises the work done during the installation phase, in particular, control of the basis weight was designed to go into service as soon as the computer was installed. It uses a control algorithm compensating for the dead time from the point of view of loop stability. The initial results are encouraging.

The same principles will be extended to the control of the dryer section after development of an appropriate model.

Head box control requirements have been reviewed and the equipment required to achieve control has been discussed briefly . In addition, methods that have been used to model the head box successfully are described and discussed from both an analytic and practical point of view. Finally, six head box control schemes varying in complexity from a simple analog level control to a complex decoupled 3 x 3 controller (controlling head box total head, liquid level and flow from the box) are described and fully discussed. The paper is closed by a brief discussion of work that ought to be done in the relatively near future.

The head box on No . 1 papermaking machine at Empire Paper Mills is an Escher Wyss pressurised type, which is run by direct digital control from the digital control computer. Control outputs are to air valves, thin stock by-pass valves and slice.

The head box stock level is maintained at α mean value by the air pressure within. The main stream flow into the head box by α coarse/fine by-pass system under DDC from the main stream magnetic flow meter. The total head now takes up some value as α dependent variable.

One of the problems encountered during the computerisation of No. 6 machine at Dartford was the need for α flow box control system to replace the existing Hornbostel hole arrangement. The circumstances were such that we required α control system implemented by the computer program (DDC) and the facilities for bumpless transfer to an analog controller arrangement in the event of computer failure.

To attain reliability and simplicity, we limited our investigations to the manipulation of the stock and air pressure supply valves to effect automatic control. Our control objectives were

1. The minimisation of level and total head variability.
2. The maintenance of stable control at all times.
3. The inclusion of efflux ratio control, in the computer controller, to reduce general variability and aid weight changes.

The dynamics of a pressurised flow box can be reasonably well described by simple transfer functions, as Dr Smith has shown. Fig. 1 represents the system as multi-variable (two input/two output).

The papers on this subject have described total head controlled by pneumatic valves as actuators. In the Chao & Sanborn paper, the authors mention that a time lag of 5 s or more must be tolerated when using these actuators. Ι suggest that somewhat more rapid control can be arranged by using a modern electronic controller.

A numerical optimisation technique has been used to show, computationally, that simple control trajectories can give considerably reduced grade change times. The models studied include one-pump and two-pump systems with and without changing wire rejection factors. Consideration has been given to some simple constraints.

Perturbation experiments on a Fourdrinier papermachine have confirmed that its dynamic behaviour can be represented adequately by a state space model in the form of a matrix difference equation. The basic equations involved have been treated in general terms, but the discussion on the model building and control system design is made more explicit by reference to a specific system.

Methods have been developed for investigating and describing the papermaking system as a process to which modern control theory can be applied. It has been shown how the model can be used to determine possible control strategies to change grade in such a way that t e grade change time is at a minimum and certain papermaking criteria are obeyed. The control objectives have been stated by analytical performance criteria in the form of quadratic cost functions.

Α simple grade change at constant machine speed was achieved by altering the thick stock flow according to a trajectory determined by the rate constraint of the flow valve. It was found necessary to manipulate and synchronise the thick stock and thin stock flows together with the machine speed in order to change grade at constant production rate.

Based on optimum control and filter theory, an on-line controller has been designed to manipulate the thick stock flow in order to minimise the variance of the measured basis weight . The developed  formulation incorporates optimum estimation of inaccessible state variables as an implicit feature. The control action is given by a proportional term together with a memory term to account for past values of control and basis weight. The controller has been implemented on a machine and is shown to have stabilised the system considerably.

Entire papermachine control implies at least the regulation of all the product quality variables such as basis weight, moisture content, strength, opacity and porosity. Implementation of such a control scheme is generally done in a piecemeal fashion . The usual starting point is automatic control of basis weight by manipulation of stock flow. The remaining variables are on manual control.

This paper presents the Warkaus Mill computer system and is focused particularly on the paper web moisture profile control developed in our company. The aim is to make use of the possibilities given by the second and the third presses with controlled crown rolls. Special attention is directed to profile measurement data analysis techniques and an on-line continuous statistical variation partitioning method is developed . The information is then used for operator guidance control, based on a derived control law . The profile control is not yet integrated in the system, but the control parameters are being established.